The invention relates to the field of data processing systems for the exchange of data or messages between extensions or stations of one or more networks.
These systems are sometimes provided with relay stations provided with a network number and comprising, inter alia, a clock which defines a local reference time, a module intended to transmit/receive messages on a frequency which varies according to a selected time-division scheme (the schemes may vary from one station to the next) defined relative to the local reference time, and a control module which forms the messages to be transmitted and processes the messages received in order, e.g. to relay them towards other stations.
Some systems include relay stations of this type referred to as xe2x80x9cvariable frequencyxe2x80x9d relay stations, intended, inter alia, to relay messages towards other stations which may or may not be relay stations. Relay stations of this kind may be, e.g. access points (or bridges) of a wire network for the exchange of messages between stations of the wire network and mobile stations of a radio network. In this example, each relay station can communicate with several xe2x80x9cnon-relayxe2x80x9d stations with which it forms a cell. A network can therefore be broken down into several cells which communicate by means of their respective relay stations.
As a result of the fact that the relay stations have personal non-synchronised clocks, they have the disadvantage that they are independent of one another, as, whether their respective time-division evolution schemes are identical or not, some of them may lag behind, with the result that the frequency on which one of the relay stations is transmitting at a given moment is not the frequency on which another relay station is receiving at this same moment.
This means that transmission between different cells is impossible as there has been no frequency tuning between these cells. This impossibility will last, on average, all the longer the greater the number of relays required for transmission, given that there must be frequency tuning at each relay.
The aim of the invention is therefore to provide a data transmission system and a corresponding process which do not have the aforementioned disadvantage.
To this end, it proposes a system of the type described in the introduction, in which:
on the one hand, the control module of each relay station brought into a selected state is adapted to form synchronisation messages including its network number and information relating to this selected state and intended at least for certain other relay stations, at least one of the relay stations, referred to hereinafter as the xe2x80x9cmaster stationxe2x80x9d, having a control module permanently verifying this selected criterion, and,
on the other hand, each relay station (both the master station and the other stations referred to hereinafter as xe2x80x9cslave stationsxe2x80x9d) includes a synchronisation module capable, upon receiving a synchronisation message containing the same network number as its own, of extracting the status information therefrom in order to bring its relay station into the selected state.
According to another feature of the invention, the information relating to the said selected state contained in the synchronisation messages includes at least information relating to a common reference time and to a common frequency and, inter alia, representative of the time-division scheme of this common frequency.
In this case, each synchronisation module is adapted to extract the information relating to the common frequency and to the common reference time from a synchronisation message received by the transmitting/receiving module associated therewith in the relay station, then, on the one hand, to adjust the local reference time defined by the local clock to the common reference time and, on the other hand, to provide the transmitting/receiving module with the information relating to the common frequency. The relay station is then considered to be in the selected state, on the one hand, when its local reference time is the common reference time and, on the other hand, when its transmitting/receiving module is ready to transmit/receive messages on the common frequency varying according to the time-division scheme of the latter.
By definition, the term synchronised station shall refer to a station brought into the selected state, i.e. a station which has been synchronised with the common reference time received from a synchronising station which may be a master station or a slave station which has just been synchronised and which relays (or broadcasts) the initial synchronisation message. In other words, a master station is necessarily a synchronising station, while a slave station can only be considered to be a synchronising station once it has been synchronised.
In this manner, by virtue of a synchronising station according to the invention, several cells independent of one another can be converted into one super cell forming a global network in which the messages can pass through from one cell to the next (or possibly from a segment of a first network to a segment of a second network).
These can be radio network segments (or cells) or radio and wire network segments, the formats of which are preferably selected, in the case of radio networks, from at least the formats of the xe2x80x9cHIPERLANxe2x80x9d standard and IEEE standard 802.11 and, in the case of wire networks, from at least the ISO standards for the IEEE standards 802.3, 802.5 and 802.14.
The HIPERLAN format is described, inter alia, in the publications of the European Telecommunications Standards Institute (ETSI) and, more particularly, in xe2x80x9cTechnical Standard ETS-300-652xe2x80x9d.
In the case of IEEE standard 802.11, the invention allows for the synchronisation of the access points (xe2x80x9cAccess Point APxe2x80x9d), such that they can use the radio interface as a distribution system (xe2x80x9cDistribution System DSxe2x80x9d) in order to communicate with one another. Extended radio networks (xe2x80x9cExtended Service Set ESSxe2x80x9d) using solely radio interfaces can therefore be obtained by virtue of the invention.
The system is moreover particularly suitable for networks referred to as xe2x80x9cfrequency-hopxe2x80x9d networks or xe2x80x9cdirect-sequence spectrum spreadingxe2x80x9d networks, well known to the person skilled in the art.
According to yet another feature of the invention, the master (or synchronising) and slave relay stations are preferably connected together, i.e. there is a path between two arbitrary stations formed by stations within radio range.
The common frequency, its time-division evolution scheme and the common reference time are preferably the local parameters of the synchronising (or master) station. However, these could of course be partially or completely different parameters.
The information contained in the synchronisation message and relating to the common frequency advantageously comprises the list of frequencies and the respective durations of these frequencies or an identifier for the list of frequencies in its evolution scheme and the respective durations of these frequencies.
According to yet another feature of the invention, each synchronising station can place in the synchronisation message an identifier intended to signal to the slave relay stations and possibly to other recipient synchronising stations the type of message it is transmitting thereto, i.e. the fact that it is a synchronisation message. In order to make use of this information, each transmitting/receiving module is capable of detecting the presence of the identifier, such that only the synchronisation messages containing it are transmitted directly to the synchronisation module for the extraction of the information they contain.
In one particularly advantageous embodiment, the transmitting/receiving modules of at least the slave relay stations are adapted to be adjusted successively to the different frequencies provided by their respective time-division evolution schemes for predetermined periods so as to be able to pick up any synchronisation message from the synchronising station (master or synchronised slave station).
These operations for successive adjustment to different frequencies (or scanning) are preferably effected when the station in question is put into operation or after each modification carried out at a station, relating, e.g. to its time-division evolution scheme or to its network number if it has one.
According to yet another feature of the invention, when the aim of the system is to connect different networks together, even only momentarily, it can advantageously assign network numbers to its different master and slave relay stations, e.g. when they are put into operation.
In this case, each transmitting/receiving module is advantageously adapted to detect the network number contained in a synchronisation message, such that only the synchronisation messages containing a network number identical to the one detected are transmitted directly to the synchronisation module for the extraction of the information contained therein. All of the other synchronisation message can thus be rejected without further processing, as a result of which the relay station can be freed for other tasks.
The synchronisation messages are preferably transmitted spontaneously, and more preferably periodically, as once the time-division evolution scheme of the common frequency is known, either the relay station in question remains synchronous with the other synchronised stations, and consequently there is no longer a problem, or the clocks of the synchronised station drift relative to one another. Spontaneous and periodic transmission of synchronisation frames is consequently particularly advantageous as a result of the fact that synchronisation between the synchronised stations can be maintained, thereby also optimising the synchronisation period for a new station.
In the case described hereinabove, the synchronisation module of each synchronised relay station (both master and slave relay station) is capable, when a synchronisation message is received from a relay station, on the one hand, of comparing its local reference time with the common reference time contained in this message and, on the other hand, of automatically readjusting its local reference time to the common reference time if the former lags behind the latter.
According to yet another feature of the invention, the control modules of the slave relay stations are adapted to form polling messages including at least their network number and requesting the transmission of a synchronisation message by the synchronising station (master or synchronised slave station). It will be clear that the control module of the master station and the slave stations must to this end be adapted to process polling messages of this kind and in return to form a synchronisation message comprising the network number of the transmitting slave relay station for the transmission thereof towards the latter. A message of this kind can also be accompanied by the identifier for the requesting station.
Other options can also be envisaged, separately or in combination:
the (master) synchronising station may be adapted to place in the synchronisation message information specifying that it is in fact the synchronising station permanently in the selected state;
the system may comprise designating means capable of defining the status of the different relay stations, namely master (or initially synchronising) station or slave relay station. In this case, the designating means may be capable of designating a slave relay station as an auxiliary synchronising station brought into the selected state, such that it can replace the synchronising (or master) station in the event of a breakdown.
This designation is preferably effected when the relay station in question is put into operation.
The replacement may be automatic if there is no synchronisation message from a synchronising (or master) station for a period greater than a selected threshold.
As a variant, replacement may be decided upon by the designating means if there is no synchronisation message from a synchronising station for a period greater than a selected threshold. In this case, the auxiliary synchronising station is preferably capable of forming and transmitting the synchronisation messages as soon as it is designated;
the master (or synchronising) station and the synchronised slave stations may be adapted to place selected complementary data in the synchronisation message.
The invention also proposes a process for the transmission of messages (or data) between master and slave relay stations of the type described hereinbefore, including the following steps:
1) assigning a local reference time to each station;
2) relating the time-division scheme of each station to its local reference time and applying this time-division scheme to the transmission/reception of this station,
the process being remarkable in that it moreover comprises the following steps:
3) forming at a relay station brought into a selected state a synchronisation message including information relating to a common frequency and, inter alia, representative of its time-division scheme, and of a common reference time, and possibly a network identifier,
4) then transmitting the synchronisation message via this station in the direction of at least one recipient relay station,
5) extracting from the recipient station the information relating to the common frequency contained in the synchronisation message,
6) adjusting the local reference time of this recipient station to the common reference time and applying the time-division scheme of the common frequency to the transmission/reception of the recipient station so as to bring the said recipient station into the said selected state, and
8) authorising the recipient station brought into the selected state to repeat steps 3) and 4).
This process may also comprise complementary steps, such as, inter alia, those in which the relay stations transmit polling messages if they have not received a synchronisation message for predetermined periods.